This invention relates to a vehicle guidance system, and particularly but not exclusively to a vehicle guidance system for an unmanned personal rapid transit (PRT) vehicle.
In general, a personal rapid transport (PRT) system comprises a dedicated track on which individual vehicles travel between stations. Each vehicle contains only one passenger or group of passengers, and the vehicle travels continuously between the starting point and the destination without stopping at any intermediate stations. PRT systems thus provide a compromise between a conventional mass transport system such as buses, trains and metro systems, and individual passenger cars.
It is known to provide an unmanned vehicle or driverless vehicle suitable for travelling on rails, such as monorail or dual rails used in conventional railway construction. The railway track is usually part of a network of track, and to travel from one destination to another, points at the junctions between tracks have to be arranged accordingly. The vehicles follow the tracks without any on-board steering control and cannot make a journey to a destination in the network, without the points being arranged specifically for that journey. An example of such a system is disclosed in U.S. Pat. No. 5,778,796.
It is also known to provide an unmanned self-steering vehicle with sensors that follow a track, e.g. a foil, rail or marked surface set into or resting on the ground. Typically, the vehicle slavishly follows the track, and is limited to the route specifically provided for by the track.
According to the present invention there is provided a vehicle guidance system for directing a vehicle along a predetermined path along which extends an elevated structure having a reference surface, the system comprising a control means, a memory for storing the predetermined path, and a sensor which is adapted to measure the distance, laterally of the vehicle travel direction, between the sensor and the reference surface provided along the predetermined path, the control means being adapted to control a steering system of the vehicle to correct any lateral deviation of the vehicle from the predetermined path in response to an output from the sensor.
It is an advantage of the invention that the memory can store a number of predetermined paths, one of which can be selected by a user travelling in the vehicle.
It is a further advantage of the invention that the vehicle guidance system can be used in a vehicle for use in any travel network, by storing the predetermined paths specific to the desired network in the memory.
The term “steering system” is intended to include any type of mechanism, device or control device, which is capable of steering or directing a vehicle along a path in response to an output or control signal from a control means, including a computer. In a preferred embodiment, the vehicle is a wheeled vehicle and the steering system comprises steerable wheels and a steering mechanism that controls the steering direction of the steerable wheels.
The sensor may be one of a plurality of sensors. For example, four sensors may be provided, each positioned substantially at or near a corner of the vehicle, and directed laterally of the longitudinal axis of the vehicle. Preferably two sensors are provided on each side of the vehicle, which are spaced apart sufficiently to detect any angular change in the position of the central axis of the vehicle relative to a substantially straight reference surface.
The or each sensor may be a non-contact sensor, such as an ultrasonic, laser, radar or any other suitable sensor for sensing distance.
The vehicle guidance system may comprise a trackway on which the vehicle runs. The trackway may be bounded by curbs laid on each side of the trackway. The curbs may provide the reference surface, which is sensed by the sensors. The term “trackway” is intended to include any track, roadway or surface, dedicated or otherwise, over which the vehicle may run. In a preferred embodiment, the system comprises a trackway having a generally flat running surface for the vehicle, bounded laterally by curbs or other elevated structures. The trackway may for example, be constructed in accordance with British Patent Specification 2384223.
The trackway may be banked or super-elevated at bends in the trackway, determined by the intended operating speed of the vehicles and the layout of junctions.
The sensors preferably detect the distance, laterally of the vehicle travel direction, between the sensors and the curb sides or trackway edges.
The or each sensor may operate continuously to sense the distance from the sensor to the curb side, enabling the computer to continuously correct for any lateral deviation from the predetermined path. The term “continuously” is intended to include both continuous analogue operation of the or each sensor as well as digital operation at a relatively high sampling rate.
The curbs may be interrupted at junctions, enabling the vehicle to move between trackways.
The difference or error between the actual position of the vehicle as detected by the sensors, and the ideal position of the vehicle as determined by the predetermined path may be used to correct both the lateral deviation of the vehicle from the predetermined path and also the vehicle alignment with predetermined path.
Reference markers may be positioned at intervals along the predetermined path. The reference markers may be reflectivity changes in protuberances or undercuts provided in the curb sides, which may be identifiable by the sensors as a step discontinuity in the distance signal output by them. The control means may compare the actual position of the vehicle on the predetermined path, as established by means of a reference marker, with an estimated position derived from the stored predetermined path by means of travel distance measuring means such as an odometer, and may reset the position of the vehicle along the predetermined path accordingly.
The control means may check the integrity of the distance values to the curb sides provided by the sensors by comparing the sensed trackway width at an estimated position along the predetermined path with the stored trackway width, which may vary along the predetermined path. If the error between the sensed trackway width and the stored trackway width is within a given tolerance, then the sensor measurements are considered valid.
If the error is outside the given tolerance, then the control means may compare the sensed distances of each sensor with stored distances. If the error for a given sensor is within a given tolerance, then that sensor measurement is treated as valid. If not, then the measurement for that sensor is treated as invalid. Any invalid measurements identified may be ignored by the control means and the steering controlled on the basis of the valid measurements obtained and stored information.
An advantage of this feature is that anomalous sensor outputs, resulting for example from rubbish or other material accumulating on the trackway, are ignored, and the operation of the vehicle guidance system is unaffected.
One or more transponders, which may be passive transponders, may be positioned in the predetermined path, at locations stored by the control means. If the control means is unable to determine its actual position along the stored predetermined path, then the control means may control the vehicle to move at a reduced speed along the trackway. When a transponder is reached, a reader on the vehicle may receive an identifying signal from the transponder, from which the control means is able to identify the actual position of the vehicle along the predetermined path on the trackway.
This feature thus provides the further advantage that if a vehicle becomes temporarily lost, then by the use of the transponders provided at known positions, the control means is able to re-establish the position of the vehicle and resume travel at normal operating speed to the required destination.
A further control means may also be provided remote from the vehicle, which may be in wireless communication with the onboard control means of the vehicle. The remote control means may control or partially control the onboard control means, thereby enabling remote control of the vehicle. The further control means may be capable of over-riding the control of the onboard control means to start or stop the vehicle, for use, for example, in an emergency by a system operator who is overseeing the smooth running of a network of vehicles.
The invention will now be described, by way of example, with reference to the accompanying drawings, in which,